The GH3 cell line has been used extensively to study prolactin (PRL) and growth hormone (GH) secretion. Information from these studies has provided insight into the process of hormone secretion from normal pituitary cells. GH3 cells can be maintained in culture for long periods of time without losing the capacity to secrete GH and PRL. Also, cultures of these cells do not contain other types of secretors that are found in preparations of normal pituitary cells. These and other characteristics enable experiments to be performed that are too difficult or impossible to conduct using normal pituitary cells. Additionally, it has been suggested that individual cells in GH cultures secrete both GH and PRL. This is very important in light of the recent observation that cells which secrete both GH and PRL are also present in cultures of normal pituitary cells. Thus, GH3 cultures provide a model that can be used to assess the importance of different cell types in the secretion of these two hormones. Recent experiments in our laboratory demonstrate that chronic hormonal treatment which reciprocally alters GH and PRL secretion from entire cultures of GH3 cells also induces a reciprocal shift in the proportion of GH and PRL secreting cells in culture. These findings indicate that an interconversion of GH and/or PRL secreting cell types may be important in the modulation of hormone secretion in GH3 cultures and raise the possibility that a similar process may be involved in the physiological regulation of GH and PRL secretion. Therefore, the purpose of this study is to identify the types of PRL and/or GH secretors in GH3 cultures and to determine whether an interconversion of two cell types may contribute to changes in hormone secretion. I plan to accomplish these objectives by using the reverse hemolytic plaque assay, which enables identification of hormone secretion from individual cells to establish the types of hormone secretors in culture. I will then monitor individual cells during chronic hormonal treatment to determine whether an interconversion of one cell type to another occurs. The proposed studies will provide an answer to some very basic questions about the involvement of changing cell types in the modulation of GH and PRL secretion. Moreover, this information can be used for more insightful design of future studies that may lead to eventual understanding of the cause and treatment of pituitary abnormalities, such as microadenmas, or the treatment and control of such pathological conditions such as galactorrhea-amenorrhea or dwarfism.